Epstein-Barr Virus (EBV) is a human herpes virus that infects over 90% of the world's population (Rickinson, 2007, Epstein-Barr Virus., 5th ed., Wolters Kluwer Health/Lippincott Williams & Wilkins, Philadelphia and Kieff, 2007, Epstein-Barr Virus and its replication, 5th ed., Wolters Kluwer Health/Lippincott Williams & Wilkins, Philadelphia). Primary EBV infection is the major cause of infectious mononucleosis. However, EBV persists as a latent infection in, and drives formation of, many lymphoid and epithelial malignancies, including Burkitt's lymphoma (BL), Hodgkin lymphomas, nasopharyngeal carcinoma (NPC), and gastric carcinoma (GC) (Thorley-Lawson, 2005, J. Allergy Clin. Immunol. 116:251-261, quiz 262; Klein, 2007, Oncogene 26:1297-1305; Raab-Traub, 2002, 12:431-441; and Fukayama, 2011, Pathol., Res. and Prac., 207:529-537). Latent infection with EBV is a major cause of post-transplant lymphoproliferative disease in immunosuppressed patients (Loren, 2003, Bone Marrow Transplantation, 31:145-155 and Gottschalk, 2004, Post-Transplant Lymphoproliferative Disorders, Annu. Rev. Med.) and greatly enhances risk of developing non-Hodgkin and primary CNS lymphomas in the HIV-positive population (Hamilton-Dutoit, 1993, Am. J. Pathol., 143:1072-1085 and MacMahon, 1991, Lancet 338:969-973). Most EBV associated cancers contain viral DNA that exists predominantly as a latent infection in which only a limited set of viral genes are expressed (Young, 2004, Nature Rev. Cancer 4:757-768). These latency associated genes are implicated in host-cell proliferation and survival, and latent EBV can directly promote tumor progression.
Current chemotherapeutic treatments of EBV-positive cancers include broad spectrum cytotoxic drugs that ignore the EBV-positive status of tumors. A recently proposed approach to treat EBV-positive cancers involves the induction of EBV lytic cycle followed by administration of antiviral drugs (Gutierrez, 1996, Cancer Res. 56:969-972; Kenney, 1998, Hum. Gene Ther. 9:1131-1141; and Moore, 2001, Antimicrob. Agents and Chemo. 45:2082-2091). This targeted “oncolytic therapy” requires the initiation of EBV lytic cycle and expression of viral kinases, which phosphorylate nucleoside analogues, e.g., gancyclovir (GSV), by converting the pro-drug to an active, selective suicide substrate for the viral and cellular DNA polymerases. This strategy aims to lower side effects associated with standard chemotherapy presently used to treat lymphomas and provides a molecular targeted therapeutic strategy by exploiting the biology of a key etiologic disease factor.
Several limitations of viral “oncolytic therapy” exist. For example, most existing methods activate/reactivate the lytic life-cycle in only a low percentage of latently infected cells. Many of the antiviral drugs are toxic; and most of the viral vectors are cell-type restricted or cell line specific. In addition, out of many known chemical activators of the EBV lytic cycle, only the histone deacetylase inhibitors derived from butyrate analogues have been tested in clinical trials (Perrine, 2007, Blood 109:2571-2578; and Ghosh, 2012, Blood, 119:1008-1017). In one clinical trial, arginine butyrate was found to be efficacious but was not tolerated due to toxicity, while sodium butyrate (NaB) was found to have unsuitable pharmacokinetics. More recent studies have screened clinically approved drugs for potential activators for latent EBV and identified bortezomib as an activator of latent EBV in a limited number of EBV positive BL cells (Shirley, 2011, Blood 117:6297-6303).
To date, no single EBV activator consistently reactivates EBV in all EBV positive cell lines (Miller, 2007, Adv. Cancer Res. 97:81-109 and Daigle, 2011, J. Virol. 85:5628-5643). Nor have any small molecules, including histone deacetylase (HDAC) inhibitors, proven safe or effective in clinical trials for treatment of EBV positive cancers.
What is needed in the art are small molecule compounds which active latent EBV and are effective in treating EBV positive cancers.